Air discharge valves of the kind defined above are typically used in water or sewage transport systems to permit air to be vented from pipelines or holding vessels to atmosphere during normal operation of the systems but to prevent spillage of liquid from the systems in the event of pressure surges in the liquid in the system or other system malfunction resulting in liquid being forced from the system into the valve. The air normally passes from the system into the valve assembly and then out of the valve assembly through one or more vents or discharge openings. A movable closure member forming part of the assembly is actuated by any system liquid which tends to pass out through the discharge opening. Basically, such a valve assembly includes a casing or housing which is in communication via an inlet at its lower end with the pipeline or vessel and in communication with the atmosphere at a location above the inlet via the air discharge opening. The movable closure member for the discharge opening is located in the casing and is connected to or part of a hollow, sealed float, also located in the casing. In the event that liquid enters and rises in the casing from the liquid transport system the float becomes buoyant and rises with the rising liquid. The closure member is moved by the float into engagement with the discharge opening and thereby prevents spillage of liquid through the air discharge opening.
The object of the present invention is to provide a float-operated air release valve which has decreased response time to potential liquid spillage from the valve, without increasing the size or complexity of the valve. A decrease in response time is particularly desirable in raw sewage systems in order to ensure that raw sewage is not permitted to be discharged. This object is achieved by means of an improved float having a downwardly facing impact surface located so as to be immediately impacted by liquid rising in the casing. The impacting force of the liquid on the impact surface of the float causes the float to begin moving upwardly before buoyancy alone would be effective to cause movement and thus the air discharge opening becomes closed very quickly. It has been found that by providing an impact surface in the form of a downwardly facing concave surface of a substantial area relative to the cross sectional area of the float, upward movement of the float and hence closing of the discharge opening is significantly more rapid than with conventional floats which have lower ends which are downwardly convex, usually hemispherical in shape. More specifically, it has been found that a concave impact surface is more effective in reducing the response time of the valve assembly than is a flat surface which is perpendicular to the direction of liquid movement. The degree of concavity should be 50%-200%, preferably 70%-100%, according to the formula (D.sub.c /D.sub.f).times.100=percent concavity, where D.sub.c is the diameter of curvature of the concave surface and D.sub.f is the diameter of the float.
In the preferred construction the concave surface of the float bottom occupies 75% or more of the transverse cross sectional area of the float. In functional terms the concavity should be such that it provides an impact area sufficient to produce a response time significantly less than the response time of a flat float bottom.
It has also been found that providing a downwardly projecting flange extending around all or a substantial portion of the concave impact surface is significant for effecting reduced response time. The ratio of flange length to float diameter should be at least about 1:20 and may be up to about 1:5, e.g. for a float of 5 inch diameter the flange should be 1/4 inch to one inch in length.